The proposed research will deal with three aspects of descending control of somatosensory input and it involves both anatomical and electrophysiological techniques. In project 1, we propose to trace projections from functionally different cortical regions to the dorsal column nuclei (DCN) of cats and macaques. This will be achieved by injection of tritiated aminoacids in cortical subfields (areas 4, 3a, 3, 1 and 2) identified and characterized by single unit recording. In the same animals, prior to sacrifice, single unit recordings from the DCN will supply a scheme of DCN functional organization plotted in terms of cytoarchitecture, upon which results from autoradiographic preparations will be superimposed. For projects 2 and 3, a double (retrograde) tracer approach will be used. Such an approach is based upon the simultaneous injection of HRP and 3H-apo-HRP(or 3H-wheat germ agglutinin) in two different CNS structures and visualization of the two tracers by histochemical methods (for HRP) and by autoradiography (for 3H-apo-HRP). In project 2, the question of whether or not at least part of cortico-DCN projections in cats and monkeys are represented by branches of cortico-spinal axons will be answered. In project 3, a comprehensive investigation will be carried out also in cats and monkeys to identify the location and relative proportion of neurons in different supraspinal centers which, by way of collateral branching, may establish synaptic connections with neurons at cervical and lumbar spinal levels.